Transversal equalizers have long been used to compensate for the time-varying distortion introduced during the propagation of digital data through a transmission channel. The transversal equalizer comprises a tapped delay line, multipliers for multiplying the digital signal in each tap with a tap-weight coefficient, and a combiner which sums the product formed by each multiplier. Adjustment of each tap-weight coefficient to its optimum value can be accomplished by a variety of techniques. The term "optimum value" herein shall be understood to include some specific value or this specific value plus or minus some small coefficient error.
In automatic equalizers, convergence of the tap-weight coefficients to their proper values is provided by the use of a training period wherein known sequences of digital data are transmitted and the coefficients are adjusted based on an error signal. This error signal is equal to the difference between the summed delay line tapped outputs and the expected data values. In adaptive equalizers, the coefficients are continuously adjusted based on the received data whose values are not known apriori, but are estimated by a quantizer which assigns the equalized signal to the closest one of the ideal digital signal levels. Adaptive equalizers can, of course, also utilize a training period to initially set the tap-weight coefficients to their proper values.
The use of a training period to adjust the tap-weight coefficients causes difficulty in noisy transmission channels because the error signal includes a component due to improperly set tap-weight coefficients and a component due to noise. Specifically, while the date sequence is known, it is not known how much of the difference between the received and expected data is due to improperly set tap-weight coefficients and how much of the difference is due to noise in the transmission channel. This distinction is of import since noise is a random and rapidly varying phenomenon whose magnitude has a zero average over time. Accordingly, adjusting the tapweight coefficients in response to the error signal component due to noise is improper and increases the time required for the tap weight coefficients to converge to their optimum values and the resulting coefficient errors.
One technique used to improve the convergence process in noisy transmission channels is to reduce the gain in the equalizer during the training period. While this technique lessens the step-size adjustment of the tap-weight coefficients in response to any sample, and hence, can provide acceptable coefficient errors, the time required for convergence of the coefficients to their optimum values is significantly increased. In many telecommunications applications, this increase in convergence time exceeds system performance objectives. Accordingly, a scheme which shortens the time required to adjust the tap-weight coefficients of an equalizer to their optimum values during their training period and still provide acceptable coefficient errors would be desirable.